The present invention relates to fault indicator circuits and, more particularly, to fault indicator circuits for detecting and responding to failure of a transistor in a switching power regulator.
Electronic fault indicator circuits have been applied to switching power regulators wherein the switching power device is a silicon controlled rectifier (SCR). In such SCR circuits, power is regulated by time ratio control of the SCR and the fault indicator circuit typically measures the impedance of the SCR or the time period for which it is conductive. If the conduction time period exceeds some predetermined limit, a fault indication is provided. A typical example of such a fault detector circuit is shown in U.S. Pat. No. 3,914,675 issued Oct. 21, 1975, and assigned to General Electric Company.
Other types of fault indicator circuits depend upon an excess of current flowing in the switching devices in order to trip an overcurrent device. Such overcurrent devices may comprise overcurrent relays of a type well known in the art or static fuses. However, in some applications a fault may not be evident by excessive current. More particularly, a fault in a component of a control circuit may result in misoperation of some other portion of the system without any indication of a fault existing in the control itself.
An example of a system in which a control fault does not create an overcurrent condition in the control but does create misoperation of another part of the system is in a control system for an electrically powered vehicle such as, for example, a fork lift truck. In such a system, a plurality of electomechanical switches are provided to prevent operation of the vehicle unless an operator is seated upon the driver seat to thereby enable a seat mounted switch, a key switch has been turned to an ON position, and a direction control lever has been placed in either a forward or reverse position. All of these electromechanical switches typically provide control signals which allow a solid state device such as a transistor to be gated into conduction and thereby provide power to energizing relay coils which in turn permit power to be applied to the drive motors of the vehicle. However, if the solid state device has failed in a short circuit condition, none of the aforementioned electromechanical switches will be effective to restrict the application of power to the vehicle.
It is an object of the present invention to provide a fault indicator circuit for use with a transistorized power regulator for indicating failure of the transistor regulator.